This is several books in one, and most of those books are really
rather good. Niven does a great job of the sense-of-wonder that's a
key part of good science fiction: the ring is huge, half a million
miles wide, with thousand-mile-high mountains camouflaging the rim
walls, and oceans large enough that you can build full-scale maps of
planets on them… and, if you're not careful, lose them.

But this is meant to be a puzzle story: who built it? Why? Where are
they now? And so the civilisation that built the thing has fallen,
with various automatic systems still operating but nobody around who
can explain anything. This is one of the classic macrostructure
stories (along with Rendezvous with Rama and Orbitsville), and all
three writers decided that having someone around who could answer
the questions would be less interesting than having the protagonists
try to work things out for themselves. (These must surely have been an
influence on the young Jack McDevitt.)

And then there's the matter of the explorers: a mad Puppeteer (being
descended from herbivores, their usual reaction to danger is to flee,
but Nessus is much braver than they consider normal), a civilised kzin
(carnivorous aliens who fought humans and were nearly wiped out; they
always attack before they're ready), Louis Wu the viewpoint character,
and Teela Brown. She doesn't seem to have any particular skills,
except that the Puppeteer who's organising all of this thinks that she
is lucky: literally, as a descendant of several generations of
Birthright Lottery winners. (This tactitly assumes, of course, that
having offspring is always a good thing for the haver.)

Unfortunately the most alien of these, as portrayed, is the human
woman. Perhaps because of her luck (if it exists), or just because
she's young and living in a well-ordered civilisation, she's never
encountered significant hardship; in fact she comes across as a
California hippie of the "it'll all be OK, sweet summer child" type.
And then there's the sexual politics: both Teela and another female
character introduced later are basically clueless (sometimes with deep
knowledge in a particular area that they don't know how to use), and
thoroughly in lust with Louis Wu. (Who's 200 years old, by the way,
and Teela's twenty. Sure, there's no physical incompatibility thanks
to medical miracles, but that just makes the mental incompatibility
more obvious; he's a hedonistic idiot who needs constant sex and
ego-stroking.) Essentially, being female is regarded as a defining
character trait (like "being a kzin" or "being a Puppeteer"), which
substitutes for having an actual personality; Teela's entire character
arc is supposedly about her becoming a less-naïve person, but is
actually about her finding the right man to be with. (And, as Niven
all but admits, breaking the story in order to make this happen; in
the hands of another writer this could have been the point of the
book.)

It's nearly half-way through the book before the expedition actually
lands (or rather crashes, thanks to an immensely stupid spacecraft
design) on the Ringworld, and actually the pre-crash parts work
better: establishing something about the people, seeing the Puppeteer
worlds, working things out with limited long-range observation. Once
the crew is down it essentially becomes one of those treks across an
unknown puzzle-world that Poul Anderson did so often – and, one should
admit, so well – in the 1960s and 1970s; there are remnant cultures,
but less time is spent on the survivors than goes to examining
artefacts.

Technology is of its time: magic fusion drives can develop 200
gravities for days on end without running out of reaction mass,
there's reactionless thrust and artificial gravity, and the computer
that can translate unknown languages is known as the ship's autopilot,
because that's its primary job. The nominal reason for building the
Ringworld (population pressure) makes no sense when you consider that
a human population can double in fifty years, and the Ring only has
about three million times the surface area of a planet; after a
thousand years, it'll be as full as the planet was. (Not to mention
the logistics of moving planetary populations there in the first
place.)

There's little resolution; several of the big questions don't really
get answered, and while the expedition manages to get away it's not at
all clear what's going to happen to the principal characters. Niven
returned sporadically to the Ring in later years, but unfortunately
failed to make the answers as interesting as the questions.